In recent years, a capsule endoscope constituting a swallowable endoscope having an imaging function and a radio communication function has been proposed in the field of the endoscope, and an in-vivo information acquiring system for acquiring image data in the subject (an example of the in-vivo information) imaged by the capsule endoscope has been developed. In the in-vivo information acquiring system, the capsule endoscope is an example of the mobile unit movable in the subject for transmitting a radio signal including the data in the subject, and during the period before being naturally discharged from the subject after being swallowed by way of the mouth of the subject for observation (examination), moves in such organs as the stomach or the small intestine in accordance with the vermiculation thereof, while at the same time imaging the interior of the subject at predetermined intervals of, for example, 0.5 second.
While the capsule endoscope moves in the subject, the image data picked up by it is transmitted outside sequentially by radio communication, and received by the receiving apparatus through any one of a plurality of receiving antennas arranged dispersively outside the subject. The receiving apparatus demodulates the radio signal thus received through the receiving antenna into an image signal, and generates image data by executing a predetermined image processing on the image signal obtained. Thereafter, the receiving apparatus sequentially stores the image data thus generated (i.e. the image data picked up by the capsule endoscope) in a storage unit. A user such as a doctor or a nurse causes the image data stored in the receiving apparatus to be fetched into a work station, and the image of the interior of the subject is displayed on a display unit of the work station thereby to diagnose the subject (see Patent Document 1, for example).
When receiving the radio signal from the capsule endoscope introduced into the subject, the receiving apparatus switches to one of the plurality of receiving antennas dispersively arranged outside the subject which is suitable for receiving the radio signal, so as to receive the radio signal from the capsule endoscope through the particular receiving antenna. In this case, the receiving apparatus sequentially switches the receiving antenna for receiving the radio signal among the plurality of receiving antennas, while at the same time detecting a received electric field strength of the radio signal sequentially received through the plurality of receiving antennas. Thereafter, the receiving apparatus selects, among the plurality of receiving antennas, the receiving antenna from which the highest received electric field strength has been detected, and receives the radio signal from the capsule endoscope through the receiving antenna thus selected. By sequentially switching the receiving antenna suitable for receiving the radio signal among the plurality of receiving antennas in this way, the receiving apparatus can receive, with a satisfactory sensitivity, the radio signal from the capsule endoscope moving in the subject.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-19111